How can we integrate different programming languages?

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I have heard

Don't believe everything you hear.

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How do they integrate these languages?

Why don't you ask them, rather than some random strangers?

Mainly this is done to create a core system with a scripting capability. So you have core in c/c++ and the scripting on top of that. Scripting mainly reserved for the 3rd party users who write extensions. This pragma has been around for as long as the computing systems itself. Examples: DOS - Batch scripts, UNIX - bash scripts; etc.. Lately it has progressed from the operating systems down to an individual programs: Web Browser- Java Script; Unreal Game Engine - Unreal Script; MS Excel - VBA. It is an ongoing idea to extend an existing application with an unprecedented extensibility.

It is done by creating a BNF/EBNF syntax programs that hooks into the underlying core API(application programming interface)

Hope this explains it.

Original Post:

How do they integrate these languages?

The way you implement disparate technologies is via compartmentalization into individual packages with some sort of interface to allow the individual units to talk to one another. Typically this would be some sort of server/client relationship such as an API.

A typical example of this is connecting an application to a database. Oracle, MySql, Sql Server have a completely different language set than what the calling application uses.
The Wikipedia entry for ADO.NET opens up with ADO.NET is a data access technology from the Microsoft .NET Framework that provides communication between relational and non-relational systems through a common set of components

It isnt so simple, because you need to provide and maintain the interfaces and handle a lot of errors. So normally it is done in BIGGER projects with a lot of teams, or to provide interoperability like between some stand alone products like databases, internet servers and operation systems.

One simple and plain approach is to start some executables of one language with parameters or setting via a config files which is writing its results to another file.

So normally developing in one language needs less overhead but when you have some complete solution part you try to find some interface like networking with some protocol like xml.

But in all cases you have a bunch of severe security problems, so it isnt the choice for a "lazy coder" to copy and paste some code to get it work. ;-)

A way that is very fashionable at the moment, especially when using microservices, is to use message queues, a well known example is RabbitMQ.
See overview here: message-queue-servers[^]

If you are wondering what microservices are, read: What is Docker, and why is it so popular? · Raygun Blog[^]